Microfluidic technologies refers to a set of technologies that control the flow of minute amounts of liquids or gases through fluid transport features having small characteristic dimensions, such that the volume of fluid flowing through the transport feature is typically measured in nanoliters and picoliters. Microfluidic devices comprise a large diverse class of devices employing microfluidic technologies for the purpose of transporting and analyzing such extremely small volumes of fluid. At the smaller end of the spectrum, some microfluidic devices may also be referred to as nanofluidic devices, and the term microfluidic device employed herein is intended to include such nanofluidic devices.
Fluid transport in microfluidic devices is accomplished through fluid transport features formed in or on material layers, in the form of topological substrate features such as, for example, channels, troughs, and apertures which provide fluidwise transport and/or fluidwise communication between various features of the device by allowing the passage of fluid. Such fluid transport features typically have at least one characteristic dimension (e.g., at least one of a length, width or depth dimension of a channel or trough, or a diameter or length of an aperture, through which fluid flows) of less than 500 micrometers, more typically less than 100 micrometers. With such typical channel or trough and aperture characteristic dimensions in the region of tens of microns, devices comprising complex networks of fluidic microchannels and interconnects in organic (polymer) substrates or inorganic (e.g., silicon wafer) substrates can be defined on a microfluidic chips within the size of a few square centimeters.
A microfluidic device may be as simple as a single component used to transport a microscopic volume of fluid from one location to another, or it may be comprised of several components connected together such that all components are in fluid-wise communication. Thus a microfluidic device may be comprised of a single microfluidic component (a single component that is employed to accomplish a particular purpose) or an assembly of components (a plurality of components that are assembled in a specific order to accomplish a particular purpose). Some of the more familiar microfluidic devices that have been developed are inkjet printers (typically in the form of an integrated array of microfluidic devices for printing an array of ink drops), including drop on demand printers and continuous inkjet printers, and “lab-on-a chip” assay devices. Microfluidic devices may be employed for various purposes including mixing, transporting, and delivering specific chemical reagents (both liquid and gas) to a specific location for particular purposes including blood analysis, DNA analysis by various methods, chemical analysis, chemical synthesis, image formation, and the like.
One of the driving forces behind the development of microfluidic technology (meaning microfluidic device design and theory, engineering, and manufacturing) for chemical analysis and other potential applications is that the timescale for microscale chemical reactions is fast because of the unique physics associated with small fluid volumes and that microfluidic devices may be easily automated to do routine assay and sample preparation. Microfluidic devices employ two-dimensional or three-dimensional structures for the purpose of controlling the flow of small fluid volumes. These structures may be complex surfaces, trenches or troughs, sealed trenches or channels, and apertures or holes or other complex three-dimensional structures such as flow separators, flow splitters, flow obstructers (employed to induce mixing), valves to control fluid flow, and other various types of microscopic structures containing various features including movable members that may be employed for various purposes such as pumping fluids as well as controlling fluid flow.
Because of the extremely small dimensions involved in microfluidic devices and the presence of accelerated reactions (microscale reaction occur faster because of the unique physics associated with small fluid volumes), including corrosion reactions, microfluidic devices have unique technological challenges associated with the chemical stability and, in many cases, biocompatibility of the device. Chemical and thermal stability of the materials employed to construct a microfluidic device is required to ensure that the extremely small volumes of fluid employed in microfluidic devices are not contaminated by the device itself during use. Furthermore, the use of the properties of microfluidic fluid transport features themselves to manipulate and alter the properties of fluid itself in these microfluidic transport features (by, for example, the formation of microscale and nanoscale self assembled structures in the fluid phase as a result of the fluid transport features interacting with the fluid that is resident in the microfluidic device) may be complicated by inadvertent contamination of the fluid by the device itself leading to irreproducible results. Such inadvertent contamination complicates analysis methods and may also introduce undue bias in analysis results obtained from the microfluidic device.
In the case of all analyses of biological fluids, it is highly preferable that the surfaces of the microfluidic device be highly biocompatible as well as chemically inert and non-contaminating to both the analyte as well as any reagent employed for the biological assay. Polydimethylsiloxane (PDMS), one of the common materials employed for the fabrication of microfluidic devices, and is highly biocompatible; however, this material is also viscoelastic and not structurally rigid, thereby causing problems with some device designs. PDMS also has an extremely high permeability that allows diffusion of many substances into and through the PDMS matrix including gases, small molecules and even polymers. In other words, the PDMS matrix employed in microfluidic devices can influence the concentration of materials in the analyte because species in the analyte may diffuse directly into the PDMS device structure. The concentration gradient of chemical species that occurs at the interface between the fluid and the PDMS wall structure provides a potent thermodynamic driving force for the diffusion of species into the PDMS wall structure. The small fluid volumes employed in microfluidic devices will be strongly affected by these diffusion processes and such a situation is highly undesirable for the reliable operation of microfluidic devices.
The use of various surface modification methods including plasma treatment and the application of additional films and coatings on microfluidic devices is known. Mukhopadhyay and co-workers (Mukhopadhayay, S; Roy, S. S.; D'Sa, R. A.; Mathur, A.; Holmes, R. J.; McLaughlin, J. A.; Nanoscale Research Letters, 2011, 6:411), e.g., investigated the use of various surface modifications, (including dielectric barrier discharge surface modification in air, nitrogen plasma treatment using low pressure RF plasma, coatings of amorphous hydrogenated carbon, and coatings of Si-doped hydrogenated amorphous carbon) on microfluidic devices fabricated from polymethylmethacrylate (PMMA) to see how such treatments influenced fluid flow in the device.
Biological applications of microfluidic devices also require that any film or coating employed on such an apparatus show a high degree of biocompatibility. This is especially important if the microfluidic device is employed in analyses of viable cells and other cellular structures whose inherent properties such as enzymatic activity or specific substrate adsorption might be compromised by unfavorable compatibility reactions with the microfluidic device materials of construction. Hafnium metal, hafnium oxide, zirconium metal, zirconium oxide, tantalum metal, and tantalum oxide have all been examined and found to possess an extremely high degree of biocompatibility. Matsuno et al (Matsuno H, Yokoyama A, Watari F, Uo M, Kawasaki T, Biomaterials. 2001 June; 22(11):1253-62) found that all three of these materials were biocompatible. S. Mohammadi et al (Journal of Materials Science: Materials in Medicine Volume 12, Number 7, 603-611, DOI: 10.1023/A:1011237610299 “Tissue response to hafnium” S. Mohammadi, M. Esposito, M. Cucu, L. E. Ericson and P. Thomsen) specifically investigate hafnium and found identical results. The biocompatibility of Ta is well known (see, e.g., Robert J. Hartling “Biocompatibility of Tantalum” at www.x-medics.com/tantalum—biocompatibility.htm and reference therein) and it has been employed as a biocompatible corrosion resistant element for stents, the biocompatibility being primarily due to the thin layer of extremely chemically inert oxide that is formed on the surface of tantalum metal upon exposure to aqueous fluids in biological systems.
The chemical stability of hafnium metal, hafnium oxide, zirconium metal, zirconium oxide, tantalum metal, and tantalum oxide are also well known. Rai et al (D. Rai, Y. Xia, N. J. Hess, D. M. Strachan, and B. P. McGrail J. Solution Chem, 30(11) (2001) 949-967), e.g., provide information concerning the solubility properties of amorphous HfO2. Comparable solubility curves for ZrO2 were derived by Curti and Degueldre (E. Curti and C. Delgueldre, Radiochimica Acta, 90(9-11)(2002)801-804) based on a survey of the solubility literature of ZrO2. Betrabet and coworkers (Betrabet, H. S.; Johnson, W. B.; MacDonald, D. D.; Clark, W.A.T. “Potential-pH Diagrams for the Tantalum Water System at Elevated Temperatures”, Proc. Electrochem. Soc. 1984, 83-94) have investigated the chemical stability of in the tantalum metal-tantalum oxide system with the construction of a Pourbaix diagram. The oxides HfO2, ZrO2, and Ta2O5 are each known to have exceptionally low chemical reactivity and solubility in aqueous fluids. In addition, these three oxides—HfO2, ZrO2, and Ta2O5—are also know to have great stability in contact with organic fluids as well as nearly all gases with the exception of halogenated acidic gases like HF and HCl.
Inkjet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena. Among the many advantages of inkjet printing is its non-impact, low-noise characteristics, its use of plain paper, and its avoidance of toner transfers and fixing. Inkjet printing mechanisms can be categorized by technology, as either drop on demand inkjet or continuous ink jet. Both drop on demand inkjet and continuous inkjet printing employ a printhead comprised of a material layer and drop forming mechanisms and nozzles that are located in or on the material layer. The drop forming mechanisms, nozzles, and associated ink channels in the printhead are provided in the form of an integrated array of microfluidic devices for printing an array of ink drops.
One type of digitally controlled printing technology, drop-on-demand inkjet printing, typically provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). The actuator is also known as the drop forming mechanism. Selective activation of the actuator or drop forming mechanism causes the formation and ejection of an ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. With thermal actuators, a resistive heater, located at a convenient location, heats the ink causing a quantity of ink to phase change into a gaseous steam bubble. This increases the internal ink pressure sufficiently for an ink droplet to be expelled. The bubble then collapses as the heating element cools, and the resulting vacuum draws fluid from a reservoir to replace ink that was ejected from the nozzle. The resistive heaters in thermally actuated drop on demand inkjet printheads operate in an extremely harsh environment. They must heat and cool in rapid succession to enable the formation of drops usually with a water based ink with a superheat limit of approximately 300° C. Under these conditions of cyclic stress, in the presence of hot ink, dissolved oxygen, and possibly other corrosive species, the heaters will increase in resistance and ultimately fail via a combination of oxidation and fatigue, accelerated by mechanisms that corrode the heater or its protective layers (chemical corrosion and cavitation corrosion). It is known to those skilled in the art that the resistive heating element employed in the drop forming mechanism of a thermally actuated drop on demand inkjet printhead can fail because of cavitation processes and thermally activated corrosion processes occurring during operation of the inkjet printhead with the ink, printing fluid, or cleaning fluids employed in the printing system.
To protect against the effects of oxidation, corrosion and cavitation on the heater material in drop on demand printers, inkjet manufacturers use stacked protective layers, typically made from Si3N4, SiC and Ta. In certain prior art devices, the protective layers are relatively thick. U.S. Pat. No. 6,786,575 granted to Anderson et al (assigned to Lexmark) for example, has 0.7 μm of protective layers for a ˜0.1 μm thick heater—that is, 700 nanometers of protective layers for a ˜100 nanometer thick heater. U.S. Pat. Pub. 2011/0018938 discloses printing devices having ink flow aperture extending through a substrate, where side walls of the apertures are coated with a coating chosen from one of silicon dioxide, aluminum oxide, hafnium oxide and silicon nitride. The only exemplified coating is a 20,000 Angstrom (2000 nanometers) thick silicon dioxide coating.
A second type of digitally controlled printing technology is the continuous inkjet printer, commonly referred to as “continuous stream” or “continuous” inkjet printer. These printers use a pressurized ink source and a microfluidic drop forming mechanism located proximate to the flow of ink from the pressurized ink source to produce a continuous stream of ink droplets. Some designs of continuous inkjet printers utilize electrostatic charging devices that are placed close to the point where a filament of ink breaks into individual ink droplets. The ink droplets are electrically charged and then directed to an appropriate location by deflection electrodes. When no print is desired, the ink droplets are directed into an ink-capturing mechanism (often referred to as catcher, interceptor, or gutter). When print is desired, the ink droplets are directed to strike a print medium. Alternatively, deflected ink droplets may be allowed to strike the print media, while non-deflected ink droplets are collected in the ink capturing mechanism.
U.S. Pat. No. 1,941,001, issued to Hansell on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al. on Mar. 12, 1968, each disclose an array of continuous inkjet nozzles wherein ink droplets to be printed are formed by a printhead comprised of a material layer and drop forming mechanism and the drops are selectively charged and deflected towards the recording medium. This technique is known as binary deflection continuous ink jet.
Later developments for continuous flow inkjet improved both the method of drop formation, drop forming mechanisms, and methods for drop deflection. For example, U.S. Pat. No. 3,709,432, issued to Robertson on Jan. 9, 1973, discloses a method and apparatus for stimulating a filament of working fluid causing the working fluid to break up into uniformly spaced ink droplets through the use of transducers and a method for controlling the trajectories of the filaments before they break up into droplets.
U.S. Pat. No. 6,079,821, issued to Chwalek et al., on Jun. 27, 2000, discloses a continuous inkjet printer and a printhead with a drop forming mechanism that uses actuation of asymmetric resistive heaters to create and control the trajectory of individual ink droplets from a filament of working fluid. A printhead includes a pressurized ink source and an asymmetric heater operable to form printed ink droplets and non-printed ink droplets. Printed ink droplets flow along a printed ink droplet path ultimately striking a print media, while non-printed ink droplets flow along a non-printed ink droplet path ultimately striking a catcher surface. Non-printed ink droplets are recycled or disposed of through an ink removal channel formed in the catcher.
While the inkjet printer disclosed in Chwalek et al. works extremely well for its intended purpose, using a heater to create and deflect ink droplets increases the energy and power requirements of this device. It is known to those skilled in the art that increased energy and power dissipated in an inkjet printhead increases the possibility of printhead failure caused by thermally activated corrosion and cavitation processes that occur during the operation of the inkjet printhead in contact with the ink, printing fluid, or cleaning fluid.
U.S. Pat. No. 6,588,888, issued to Jeanmaire et al. on Jul. 8, 2003, discloses a continuous inkjet printer capable of forming droplets of different size and having a droplet deflector system for providing a variable droplet deflection for printing and non-printing droplets. The printhead disclosed by Jeanmaire comprises a plurality of nozzles and a drop forming mechanism on each nozzle comprised of an annular heater at least partially formed or positioned on or in a silicon material layer of the substrate of the printhead around corresponding nozzles. Each heater is principally comprised of a resistive heating element that is electrically connected to a controllable power source via conductors. Each nozzle is in fluid communication with an ink supply through an ink passage or liquid chamber also formed in printhead. It is known to those skilled in the art that the thermally actuated resistive heating elements disclosed as part of the drop forming mechanism can become non-functional as a result of thermally activated corrosion processes that occur when the inkjet printhead is operated in contact with the ink, printing fluid, or cleaning fluid employed in the printing system.
It is known, then, that both drop on demand printheads and continuous inkjet printheads are subject to corrosion and wear during use as a result of exposure to inks and other fluids employed in printing systems. The printhead in both drop on demand and continuous inkjet printing apparatus is in continual contact with ink and it has been found that both drop on demand and continuous inkjet printheads are degraded over time by continual contact with ink and other fluids employed in printing apparatus. For example, Beach, Hilderbrandt, and Reed observed as early as 1977 the importance of material selection in inkjet printers as it relates to corrosion and wear resistance. B. L. Beach, C. W. Hilderbrandt, W. H. Reed; IBM Journal of Research and Development, volume 21, January 1977, pp 75-80; “Materials Selection for an Inkjet Printer”. As mentioned previously, a common method to address the observed performance degradation of both drop on demand printheads and continuous inkjet printheads is to coat the printhead with a corrosion resistant and/or wear resistant layer or film. Lee, Eldridge, Liclican, and Richardson proposed the use of passivating layers to address corrosion and wear resistance in continuous inkjet printheads and found that amorphous films containing silicon, carbon, and hydrogen were effective for improving corrosion and wear resistance. The amorphous films containing silicon, carbon, and hydrogen are also called amorphous silicon carbide films, amorphous silicon carbide layers, silicon carbide, and SiC; M. H. Lee, J. M. Eldridge, L. Liclican, And R. E. Richardson Jr.; Journal of the Electrochemical Society 129(10), (1982), 2174-2178; “Electrochemical test to evaluate passivation layers: Overcoats of Si in Ink”. Gendler and Chang demonstrated the corrosive effects of ink formulations on amorphous silicon carbide layers applied onto inkjet printheads. P. L. Gendler and L. S. Chang, Chem. Mater. 3 (1991)635-641; “Adverse Chemical Effects on the Plasma—Deposited Amorphous Silicon Carbide Passivation Layer of Thermal Ink-Jet Thin-Film Heaters”. The chemical stability requirements for an inkjet printhead including the drop forming mechanism are well known to those skilled in the art. The requirements for chemical stability of the printhead include stability of the printhead under complete immersion in ink and any other additional fluid employed in the printing system such as cleaning fluids and image stabilization fluids containing polymers, dispersants, surfactants, salts, solvents, humectants, pigments, dyes, mordants, and the like that are familiar to those skilled in the art. It is known that it is highly desirable for the printhead to have immunity to the effects of both anionic and cationic contamination from diffusion processes that occur upon exposure of the printhead to ink or other fluids employed in the printing system that contain cations and anions. These requirements are applicable to all inkjet printing technologies including drop on demand and continuous inkjet digitally controlled printing technologies.
In U.S. Pat. No. 6,502,925 Anagnostopoulos et al described an inkjet printhead comprised of a material layer and a drop forming mechanism. The material layer is formed of a silicon substrate and includes a nozzle array as well as an integrated circuit formed therein for controlling operation of the print head. The silicon substrate has one or more ink channels, also called ink chambers, formed therein along the longitudinal direction of the nozzle array. The material layer also includes an insulating layer or layers that overlay the silicon substrate and the insulating layer or layers has a series or an array of nozzle openings or bores formed therein along the length of the substrate and each nozzle opening communicates with an ink channel. Each nozzle of the nozzle array is in fluid communication with an ink supply through an ink channel, ink passage, or liquid chamber also formed in printhead. The area comprising the nozzle openings forms a generally planar surface to facilitate maintenance of the printhead. The drop forming mechanism, part of the material layer, is comprised of a resistive heater element, also called a resistive heater, and at least one drop forming mechanism is associated with each nozzle opening or bore for asymmetrically or symmetrically heating ink as ink passes through the nozzle opening or bore. It is known to those skilled in the art that the material layer of the printhead, as well as the drop forming mechanism in or on the material layer, is also susceptible to chemical corrosion processes and that an additional pathway available for printhead failures involves failure of the material layer and any associated electrical circuitry as a result of corrosion of the material layer or any element thereof.
The useful life of an inkjet printhead with its associated material layer and thermal actuators or resistive heaters that are part of the drop forming mechanism is dependent on a number of factors including, but not limited to, dielectric breakdown, corrosion, fatigue, electromigration, contamination, thermal mismatch, electrostatic discharge, material compatibility, delamination, and humidity, to name a few. Accordingly, the incorporation of layers, films or coatings on the material layer of the printhead, drop formation mechanism, and liquid chamber are employed to provide a printhead robust enough to withstand the different types of failure modes described above. Various types of layers, coatings, and films have been investigated for corrosion resistance. U.S. Pat. No. 6,786,575 to Anderson et al, e.g., discloses use of passivation layers comprising silicon carbide and silicon nitride. Combinations of layers, coatings, and films, are also called combination layers, combination coatings, and combination films. Combination layers in layers, films, or coatings are layers, films, or coatings where essentially a layer comprised of one material overlays and is in contact with a second layer of a second material, the second material being of different chemical composition than the first material. Combination layers comprised of only two layers, films or coatings of two different materials are also called bilayers. Combination layers can be called trilayers when three different materials are used and overlay each other, and so on. Complex coatings may be comprised of multiple combination layers. For example, a complex film, layer or coating may be comprised of multiple bilayers or multiple combination layers, combination films, or combination coatings. Complex coatings comprised of multiple layers of different materials where at least two differentiable, chemically different materials are present are also known as stacks or laminates. Films comprised of two or more layers of different chemically distinguishable materials are also sometimes called laminates, laminate films, laminate layers, laminate coatings, multilayer films, and the like. Laminate films having at least two layers whose thickness is less than 100 nm can be called microlaminates. Microlaminates are also sometimes called nanolaminates.
Combination layers, and specifically complex multilayered films comprised of multiple bilayers have been investigated for corrosion resistance in various applications with mixed results. For example, Matero and coworkers explored the use of combination layers of Al2O3—TiO2 (also called bilayers of Al2O3—TiO2) as corrosion resistant coatings on 304 stainless steel as described by R. Matero, M. Ritala, M. Leskalae, T. Salo, J. Aromaa, A. Forsen; J. Phys. IV 9 (1999) Pr8-493 through Pr9-499; “Atomic Layer deposited thin films for corrosion protection”. Whereas Al2O3 and TiO2 alone were found to have unsatisfactory corrosion resistance, Al2O3—TiO2 bilayer structures showed improved corrosion resistance performance relative to the binary oxide films. The authors specifically remarked, however, that they observed “no clear tendency to improve performance by increasing the number of layers”. Almomani and Aita investigated the use of combination layers in the hafnia-alumina system, that is, the HfO2—Al2O3 system, for improved corrosion resistance of biomedical implants as described by M. A. Almomani and C. R. Aita, in J. Vac. Sci. Technol. A, 27(3)(2009)449-455 “Pitting corrosion protection of stainless steel by sputter deposited hafnia, alumina, and hafnia-alumina nanolaminate films”.
Combination layers have also been investigated for functions distinct from providing chemical resistant corrosion protection. U.S. Pat. No. 7,426,067 discloses atomic layer deposition of various layer compositions or combination of layers on micro-mechanical devices to provide, e.g., physical protection from wear and providing electrical insulation. Control of crystallization of zirconium oxide and hafnium oxide in laminate films of zirconium oxide or hafnium oxide with aluminum oxide interlayers to achieve atomically smooth surfaces for capacitor and interlayer dielectric applications has been discussed in the literature. Hausmann and Gordon [D. M Hausmann and R. G. Gordon in Journal of Crystal Growth, 249 (2003) 251-261; “Surface morphology and crystallinity control in the atomic layer deposition (ALD) of hafnium and zirconium oxide thin films”], e.g., reported that the minimum number of aluminum oxide layers needed to retard crystal growth between two thicker layers of hafnium or zirconium oxide was approximately 5 layers of aluminum oxide (0.5 nm aluminum oxide) between approximately 100 layers of zirconium or hafnium oxide (10 nm zirconium or hafnium oxide). Control of crystallization of hafnium oxide in laminate films of hafnium oxide with tantalum oxide interlayers to achieve smooth surfaces for capacitor applications has been discussed in the literature. Kukli, Ihanus, Ritala, and Leskela [K. Kulki, J Ihanus, M. Ritala, M. Leskela, Appl. Phys. Lett. 68(26) 24 Jun. 1996 p 3737] reported that HfO2 crystallization is observed when the thickness of the HfO2 layer in HfO2—Ta2O5 nanolaminates is greater than 10 nm.
It is desirable that inkjet printheads used for continuous inkjet printing should operate without failure for extended time periods. One type of failure described above that can require printhead replacement is related to corrosion, chemical dissolution, and optionally cavitation induced failure of thermally actuate resistive heating elements in the printhead drop forming mechanism. It is also known that other heated and unheated surfaces of the printhead such as those located anywhere on the material layer of the printhead including surfaces of integrated circuits incorporated on the printhead material layer that have the possibility of exposure to ink or other fluids used in a printing system can corrode upon exposure to the inks and fluids employed in a digitally controlled printing system. Corrosion of surfaces on or proximate to the material layer can result in the printhead becoming non-functional. It is understood by those skilled in the art that a more chemically resistant and thermally stable inkjet printhead is highly desirable and can provide substantial benefits for ease of use, equipment maintenance, and overall versatility of a printing apparatus. Chemical resistance, thermal stability and biocompatibility would further be beneficial in other types of microfluidic devices, such as lab-on-a-chip and microreactor devices. Thus, there is a need for improved coatings for microfluidic devices that are chemically resistant, thermally stable, and biocompatible.